1. Field of the Invention
The present invention relates to a semiconductor memory device, for example, an FBC (Floating Body Cell) memory device storing information by accumulating majority carriers in a floating body of each field effect transistor.
2. Related Art
In recent years, there is known an FBC memory device as a semiconductor memory device expected to replace a 1T (Transistor)-1C (Capacitor) DRAM. The FBC memory device is configured so that FETs (Field Effect Transistors) each including a floating body (hereinafter, also “body”) are formed on an SOI (Silicon On Insulator) substrate, and so that data “1” or “0” is stored in each FET according to the number of majority carriers accumulated in the body of the FET.
It is defined in an FBC memory device including N-type FETs, for example, that a state in which the number of holes accumulated in a body is large is data “1” and a state in which the number of holes accumulated therein is small is data “0”. In this case, since a potential of one word line is reduced to a negative potential in a data holding state, holes are caused to flow into the body of each N-type FET. After data is written, if the data holding state is long, the number of holes flowing into the body is large, resulting in deterioration in data “0”. It is, therefore, necessary to regularly perform a refresh operation on each of memory cells storing therein data “0” (hereinafter, also “0 cells”). On the other hand, since each of memory cells storing therein data “1” (hereinafter, also “1” cells) corresponds to the state in which the number of holes is large, the “1” cells are not deteriorated in the data holding state.
However, if a selected word line is activated during a data read or write operation, a phenomenon occurs that the number of holes accumulated in bodies of unselected memory cells connected to the selected word line gradually decreases. This is referred to as “charge pumping phenomenon”. If the charge pumping phenomenon repeatedly occurs, data “1” stored in the unselected memory cells is changed to data “0”. To deal with this disadvantage, it is necessary to perform a refresh operation for supplying holes to the unselected memory cells storing therein the data “1”.
Nevertheless, the conventional technique has the following disadvantages. A refresh operation on “1” cells and that on “0” cells are executed simultaneously on regular basis. Due to this, even if there is no need to perform a refresh operation on “1” cells, the refresh operation is performed on both “1” cells and “0” cells. In this case, deterioration in the “1” cells does not always occur simultaneously with that in the “0” cells, so that power is wasted in the refresh operation.